1. Field of the Invention
The present invention relates to a print ink containing a plurality of fluorescent coloring materials applicable to printers including inkjet apparatuses, offset printers, plotters, and line printers etc., a print ink capable of increasing fluorescence properties of a printed image using such a print ink, and an inkjet recording method using such a print ink. Specifically, the present invention provides a novel technique to improve fluorescence emission characteristics of a second fluorescent coloring material to be contained in a print ink containing a first fluorescent coloring material, where the first coloring material emits fluorescence with light of a predetermined excitation wavelength and the wavelengths of the emitted fluorescence include a predetermined fluorescence wavelength for measurement or determination.
2. Related Background Art
In recent years, various applications have been requested for ink. As such applications, in addition to formation of beautiful color images, there are proposed, for example, use of fluorescence ink for providing information (such as security information) in addition to visual information, by printing information such as characters, numbers, symbols, or bar-codes with such an ink on a recording medium and irradiating UV light of an appropriate wavelength to generate colored fluorescence from the fluorescence ink. Specifically, in a system for reading out authentication (anti-counterfeit) information or security information using an apparatus to excite fluorescence and read the emission intensity thereof, a fluorescent coloring agent is excited by excitation light of a predetermined wavelength (e.g., 254 nm) to fluoresce, and the fluorescence is determined or measured.
Regarding the coloring materials in the ink, dyes can provide a predetermined color easily but occasionally poor water-resistance, while pigments can give excellent water resistance but not the predetermined color tone occasionally. In view of the above, there is proposed ink containing both dye and pigment to obtain an ink capable of providing an image excellent in both water-resistance and color tone. For example, Japanese Patent Publication No. S60-45669 discloses a recording liquid that contains a water-soluble red dye (e.g., Acid Red 52) and a red pigment as recording agents, and a polymer dispersant for dispersing the pigment in a liquid medium.
In the mailing systems of the United States, printing with fluorescent red is common, and a dye such as Acid Red 52 (AR52), which is described in the above publication, is used as a fluorescent dye. U.S. Pat. No. 6,176,908 discloses an ink containing a fluorescent dye, a pigment and a polymer as a dispersant for the pigment, exemplifying AR52 as a fluorescent dye. It should be noted, it had been a well-known design matter long before U.S. Pat. No. 6,176,908 to adjust the final color shade according to human visual sensation by combination of dyes.
U.S. Pat. No. 6,176,908 provides an inkjet ink containing a pigment in addition to a fluorescent dye for improving the water-resistance of the ink as with Japanese Patent Publication No. S60-45669, and there are described addition of two kinds of fluorescent dyes for the known object of visible (visual) color adjustment, and also additives for improving fluorescence intensity (PMU level) to the system. Concrete ink examples of improved fluorescence intensity (PMU level) contain solvents such as water, 2-pyrrolidone, and tetraethylene glycol, and the following fluorescent coloring materials other than the pigment-related component comprised of a pigment, a polymer and tetraethylene glycol or diethylene glycol. As the fluorescent coloring materials, a combination of AR52 (0.4% by mass, 0.5% by mass to 3.0% by mass) and one of AY7, AY73, and DY96, and a combination of basic violet (RHDB) and basic yellow (BY40) are described, for example.
Japanese Patent Application Laid-Open No. H11-80632 discloses an invisible fluorescence aqueous ink containing three different fluorescent coloring materials (a fluorescence brightening agent, a yellow fluorescent dye of a coumarin derivative, and a red fluorescent dye of rhodamine-B or rhodamine-6G), and postcard printing using the ink. In the technical descriptions thereof, each of these three fluorescent coloring materials emits light under UV light irradiation to excite the other coloring material sequentially leading to final fluorescence emission having a wavelength peak at 587 nm. In this publication, however, there is no concrete description about excitation wavelengths, and the technical description is made using a result that the ink and the recorded image show the same fluorescence characteristics. Generally, water absorbs UV light, so that the fluorescence of a recorded image will be different from that of the ink used. Judging from this, the invention described in the publication lacks technical credibility.
WO 02/092707 discloses an ink that can form a dark image and also exhibit fluorescence of a predetermined color when exposed exciting radiation. The ink contains a plurality of dyes (e.g., red and yellow fluorescent dyes, a blue dye, and a black dye) as with Japanese Patent Application Laid-Open No. H11-80632, but differs from Japanese Patent Application Laid-Open No. H11-80632 in that dyes are selected such that the longer wavelength absorption band and the shorter wavelength emission band would not overlap. In this publication, the relationship between the fluorescent coloring materials is not analyzed sufficiently so that the desired fluorescence intensity cannot always be obtained.
Japanese Patent Application Laid-Open No. 2003-113331 discloses an invention for improving the fluorescence characteristics of ink in terms of the relationship between solvents and fluorescent coloring materials. In other words, Japanese Patent Laid-Open No. 2003-113331 discloses a recording ink that includes two fluorescent coloring materials of the same color (there is an example where a non-fluorescent coloring material is added), two different organic solvents (e.g., glycerin and a nonionic surfactant) which have no compatibility to each other, and pure water for dissolving these components.